1. Field of the Invention
The present invention relates to an adhesive material used for mounting bare IC chips and other electronic components on circuit boards.
2. Description of the Related Art
Insulating adhesives or anisotropically electroconductive adhesives in the form of pastes, liquids, or films are widely used as adhesive materials for the mounting of bare IC chips and other electronic components on circuit boards.
Starting materials for manufacturing these adhesives are selected with consideration for their purchase price, ease of fabrication, connection reliability, storage stability, and the like. Typical examples of starting materials selected with consideration for such factors include radical-polymerizable compounds such as bisphenol A-type epoxy resins, curing agents such as imidazole-based latent curing agents and amine-based curing agents, and thermoplastic resins such as phenoxy resins and urethane-based plastic resins. Acute biological toxicity (LD50), flammability, or the like is commonly taken into account in order to ensure that these starting materials remain safe.
Adverse biological effects are not limited to the acute toxicity caused by a one-time oral ingestion (inhalation) of a large amount of starting materials and include the poisoning symptoms, malignant tumors, and teratogenic developments brought about by long-term oral ingestion (inhalation) of minute amounts of starting materials, as well as the intense allergic reactions or the like brought about by skin contact. Despite all these, insufficient attention has so far been paid to eradicating these problems with respect to conventional adhesive materials, making it difficult to conclude that the social need for safety has been properly satisfied. In addition, environmental hormones (endocrine disruptors) that cause reproductive irregularities in living organisms (including humans) have recently become a cause for concern, but no arrangements have so far been made for manufacturing adhesive materials with full consideration for these effects.
An object of the present invention, which is aimed at addressing the above-described problems of prior art, is to provide an adhesive material particularly suited to mounting electronic components on circuit boards, wherein this adhesive material is highly safe biologically despite containing a radical-polymerizable compound, curing agent, and thermoplastic resin.
The inventors perfected the present invention upon discovering that an adhesive material that gives a negative result of the Ames test and has a PII (Primary Irritation Index), or skin irritation, value of 2 or less is extremely safe biologically.
Specifically, the present invention provides an adhesive material suitable for use during the mounting of electronic components on circuit boards, wherein this adhesive material comprises a radical-polymerizable compound, a curing agent, and a thermoplastic resin; gives a negative result of the Ames test; and has a PII, or skin irritation, value of 2 or less.
The adhesive material of the present invention is particularly suited to the mounting of electronic components on circuit boards, and is characterized by comprising a radical-polymerizable compound, a curing agent, and a thermoplastic resin; giving a negative result of the Ames test; and having a PII, or skin irritation, value of 2 or less.
The Ames test referred to herein is also known as a mutagenicity test, and is one of tests in which bacteria are used to screen chemical substances for carcinogenicity or mutagenicity. Specifically, it is a test in which a mutant strain of Salmonella or Salmonella typhimurium that lacks the ability to biosynthesize histidine is used to detect reverse mutation to produce a strain not requiring histidine (JIS K 3600 2408, JIS K 3610 1605). This mutation is the result of DNA damage, and carcinogenicity is believed to become more pronounced with increased ease of mutation. Consequently, an adhesive material having high biological safety must give a negative result of the Ames test.
The PII, or skin irritation, value is a skin toxicity index measured in accordance with the method defined by the Consumer Product Safety Commission of the USA (Draize Method; The Consumer Product Safety Commission of the USA, the Code of Federal Regulations, Title 16, Section 1500.41). A lower index corresponds to less pronounced skin irritation. In the present invention, the PII value of an adhesive material must be 2 or less. A material with a PII value greater than 2 causes discomfort in most people, and is therefore unsuitable.
The adhesive material of the present invention is such that the adhesive material in itself gives a negative result of the Ames test and has a PII value of 2 or less, as described above. As long as these conditions are met, some of the starting materials used in the preparation of the adhesive material may be mutagenic or have a PII value that is greater than 2. It is, however, preferable for all the starting materials used to give negative results of the Ames tests and to have PII values of 2 or less in order to yield an even safer adhesive material.
All the starting materials contained in the adhesive material of the present invention should preferably be non-endocrine-disruptors, particularly in order to prevent reproductive cells from being adversely affected. In this case, passive selection methods dispensing with materials known or believed to act as endocrine disruptors can be used as concrete and practical techniques for selecting non-endocrine-disruptors for the starting materials ((1) T. Colborn, C. Clement, xe2x80x9cChemically Induced Alterations in Sexual and Functional Development: The Wildlife/Human Connection,xe2x80x9d Princeton, N.J.: Princeton Scientific Publishing (1992); (2) T. Colborn, F. vom Saal, A. M. Soto, Environmental Health Perspective, 101, 5 (1993); (3) G. Lyons, xe2x80x9cPhthalates in the Environment,xe2x80x9d World Wildlife UK (1995); (4) xe2x80x9cMinistry of Agriculture, Fisheries and Food, Effects of Trace Organics on Fish, Phase II,xe2x80x9d Foundation for Water Research UK (1995).).
The radical-polymerizable compound used in the adhesive material of the present invention functions as the adhesive component of the adhesive material. Examples of such materials include unsaturated polyesters, acrylic acid esters, and methacrylic acid esters. Two or more of these may be used jointly.
Non-vinyl-based oligomers synthesized from glycols, and an unsaturated dibasic acids (maleic anhydride, fumaric acid, and the like) can be cited as examples of such unsaturated polyesters. Of these, diallyl phthalate oligomers capable of improving adhesion to ITO electrodes are preferred.
The acrylic acid esters may be acryl-based monomers or oligomers having one or more, and preferably up to four, acryloyl groups in their molecular structures. The methacrylic acid esters may be methacryl-based monomers or oligomers having one or more, and preferably up to four, methacryloyl groups in their molecular structures. Examples of preferred compounds include 1,3-butanediol methacrylate, neopentyl glycol dimethacrylate, 2-hydroxypropyl methacrylate, hydroxyethyl methacrylate, alicyclic modified neopentyl glycol acrylate, phenol ethylene oxide-modified acrylate, polypropylene glycol diacrylate, and ditrimethylol propane tetraacrylate. All these have a PII value of 0.0.
The isocyanate-derived urethane acrylate and the epoxyacrylate-based polymers (derived from bisphenol A or bisphenol F) commonly used as radical-polymerizable compounds should be dispensed with in the present invention because of their mutagenicity.
A curing agent produces dissociated radicals under the action of heat or light. An organic peroxide is preferred as such a material. A peroxydicarbonate is particularly preferred. Commonly used imidazole-based latent curing agents are mutagenic and should not be used in the present invention.
A thermoplastic resin primarily functions as the film-forming component of an adhesive material. Saturated polyester resins and polyvinyl acetate can be cited as examples of such materials.
The starting materials of the inventive adhesive material should preferably contain electroconductive particles. The adhesive material of the present invention can therefore be used as an anisotropically electroconductive adhesive. Gold particles can be cited as preferred electroconductive particles. Common nickel particles cause skin irritation and should not be used in the present invention.
Various non-mutagenic additives whose PII values are 2 or less, such as coupling agents and compounds containing glycidyl groups, may also be used as needed to obtain the adhesive material of the present invention.
The adhesive material of the present invention can be manufactured using a common technique to uniformly mix such radical-polymerizable compounds, curing agents, and thermoplastic resins.
The adhesive material of the present invention can be used in a variety of applications involving TCPs (Tape Carrier Packages), FPCs, PWBs, glass, and plastic wiring materials, and is particularly suitable for use in the mounting of bare IC chips on circuit boards. Specifically, a bare IC chip can be connected to a wiring circuit board by a method in which the adhesive material of the present invention is fed onto the circuit board on which a bare IC chip is to be mounted, the bare IC chip is positioned thereon, and the assembly is heated under pressure.